Existence of Brain 5-HT1A–5-HT2A Isoreceptor Complexes with Antagonistic Allosteric Receptor–Receptor Interactions Regulating 5-HT1A Receptor Recognition

摘要

Studies on serotonin-selective reuptake inhibitors have established that disturbances in the ascending 5-HT neuron systems and their 5-HT receptor subtypes and collateral networks to the forebrain contribute to the etiology of major depression and are targets for treatment. The therapeutic action of serotonin-selective reuptake inhibitors is of proven effectiveness, but the mechanisms underlying their effect are still unclear. There are many 5-HT subtypes involved; some need to be blocked (e.g., 5-HT2A, 5-HT3, and 5-HT7), whereas others need to be activated (e.g., postjunctional 5-HT1A and 5-HT4). These state-of-the-art developments are in line with the hypothesis that the development of major depression can involve an imbalance of the activity between different types of 5-HT isoreceptors. In the current study, using in situ proximity ligation assay (PLA), we report evidence for the existence of brain 5-HT1A–5-HT2A isoreceptor complexes validated in cellular models with bioluminescence resonance energy transfer (BRET²) assay. A high density of PLA-positive clusters visualizing 5-HT1A–5-HT2A isoreceptor complexeswas demonstrated in the pyramidal cell layer of the CA1–CA3regions of the dorsal hippocampus. A marked reduction in the densityof PLA-positive clusters was observed in the CA1 and CA2 regions 24h after a forced swim test session, indicating the dynamics of this5-HT isoreceptor complex. Using a bioinformatic approach, previouswork indicates that receptors forming heterodimers demonstrate tripletamino acid homologies. The receptor interface of the 5-HT1A–5-HT2Aisoreceptor dimer was shown to contain the LLG and QNA protripletsin the transmembrane and intracellular domain, respectively. The 5-HT2Aagonist TCB2 markedly reduced the affinity of the 5-HT1A agonist ipsapironefor the 5-HT1A agonist binding sites in the frontal lobe using the5-HT1A radioligand binding assay. This action was blocked by the 5-HT2Aantagonist ketanserin. It is proposed that the demonstrated 5-HT1A–5-HT2A isoreceptor complexes may play a role in depression through integrationof 5-HT recognition, signaling and trafficking in the plasma membranein two major 5-HT receptor subtypes known to be involved in depression.Antagonistic allosteric receptor–receptor interactions appearto be involved in this integrative process.